Got a Pi running LMS (via Max2Play) in the mancave which has a hard drive containing the music files attached via USB. Is there a way of transferring FLAC files to it that doesn’t involve powering down the PI and attaching the drive to the laptop where the ripped files are?

Oh OK, well the easiest way would be to make the USB drive show up on the network. Is the pi on the network? If so, you simply need to configure samba. If not, get it onto the network (USB WiFi stick is a few quid) and then sort samba